The invention concerns the field of optical sources, and their use as components in ultrahigh frequency emitter arrays.
Ultrahigh frequency emitters making use of optical sources are used in the field of optical ultrahigh frequency telecommunications, as described in the document by G. Grosskopf entitled xe2x80x9cOptical fibres deliver microwave broadcastsxe2x80x9d published in O.L.E., Sep. 1996, p. 55-61.
One possible application of the invention is therefore xe2x80x9cpicocellular radiotelephonyxe2x80x9d with the use of the fiber telecommunication network as the channel for transporting information to the radiating points.
The generation of ultrahigh frequency signals by optical signal heterodyning requires light sources of the laser source type, shifted frequency-wise by the value of the ultrahigh frequency sought.
This frequency shift can be obtained in different ways:
a) with two lasers, there can be obtained, if they have slightly different optical cavity lengths, two emitters with slightly shifted wavelengths, and therefore with emission frequencies capable of differing by that of the ultrahigh frequency to be generated.
This principle of the optical transportation of ultrahigh frequency signals is shown in FIG. 1.
Two optical sources 2, 4, emit respectively radiation at the frequency xcexa9 and xcexa9+xcfx89 and are coupled to a mixer 6 (a photodiode) by means of optical fibers 8, 10. The part 12, situated beyond the mixer, forms the antenna. A device 14 makes it possible also to slave the two lasers frequency-wise.
b) Within a single laser cavity, it is possible to make two modes of oscillation coexist simultaneously (with orthogonal, linear or circular polarization states), whose frequency difference can be adjusted to the value of the ultrahigh frequency sought. Such a system is for example described in the document by M. Brunel et al. entitled xe2x80x9cDifferential measurement of the coupling constant between laser eigenstatesxe2x80x9d published in Applied Physics Letters, vol. 70, no. 16, April 1997.
Other methods are known, like the use of an optical frequency shifter (typically, an acousto-optical modulator) for obtaining two sources shifted frequency-wise. This technique is not compatible with the frequencies sought (≈10 GHz).
All the known devices pose a problem of size.
Moreover, in the known devices, correction devices are necessary on account of the over-large spectral width of the optical signal.
Finally, laser diodes, used in the majority of cases, are in general modulated frequency-wise by their supply current, and therefore also, simultaneously, emission amplitude-wise.
Another example field of application of the invention is the radar field.
Modern radars use a so-called active antenna design in which the angular scanning function of the antenna is obtained not by rotation of the antenna itself but by that of its emission wave plane. The wave plane results from a phasingxe2x80x94in the direction soughtxe2x80x94of elementary waves issuing from various radiating elements of the antenna. The phasing is generally obtained by the adjustment of delays on the transport routes of the different ultrahigh frequency signals. FIG. 2 shows this principle schematically.
The radiation produced by an emitter 13 is divided into n beams each passing through means 15, 17, 19, 21 for imposing a delay on them. The wave 23 emitted by the antennas 25 has its wave plane modified according to the different delays imposed.
The necessity of having a greater and greater number of radiating antenna elements in order to improve, notably, the angular resolution (2000 emitters are spoken of), and the need to provide an accurate management of the delays, lead to a complexity of the system which is difficult to control with conventional ultrahigh frequency techniques (sizes and weights incompatible with the requirement of certain devices, airborne devices notably).
The present invention concerns the use of components in an array, in planar and collective manufacturing technologies for implementing ultrahigh frequency emitters.
The invention therefore concerns an ultrahigh frequency emitting device having a number of lasers and N means making it possible to impose phase delays on the path of N laser beams, these means being implemented in an array or a bar.
The invention concerns in particular an ultrahigh frequency emitting device, having:
at least a first and a second laser, emitting at two different frequencies xcfx891 and xcfx892,
means of slaving the first and the second laser frequency-wise,
a mosaic or a bar or an array of N elements (Nxe2x89xa72) placed on the path of the beam of the second laser, each element making it possible to impose a phase delay on the bear or the portion of beam which passes through it,
N means for mixing the beam emitted by the first laser and each of the N delayed beams, and for producing N signals of frequency xcfx891-xcfx892,
N antenna-forming means for emitting radiation at the frequency xcfx891-xcfx892,
The invention also concerns an ultrahigh frequency emitting device, having:
a plurality of N laser emitter pairs, implemented in a mosaic or an array or a bar, each laser emitter pair having a first and a second laser emitter emitting at a first and a second frequency xcfx891, xcfx892, which are different,
an array or a bar of N elements, each of them being placed on the path of the second laser emitter of one of said laser emitter pairs, and each element making it possible to impose a chase delay on the beam of said second laser emitter,
means of slaving each laser emitter pair, frequency-wise and phase-wise,
N means for mixing each of the beams emitted by the first emitters of the N laser emitter pairs with each of the beams emitted by the second emitters of the N laser emitter pairs and delayed by the elements making it possible to impose a phase delay, and for producing N signals at the frequency xcfx891-xcfx892,
N antenna-forming means for emitting radiation at the frequency xcfx891-xcfx892,
The invention is based on a principle of ultrahigh frequency electromagnetic wave generation (a frequency which may reach several hundred GHz) by means of the beatingxe2x80x94the heterodyningxe2x80x94of at least 2 electromagnetic waves in the optical domain (of much higher frequencies, of the order of 1014 Hz), generated by lasers. The use of elements in mosaics or arrays, for imposing phase delays, allows the implementation of a compact device.
The detection of frequency beats (a mixing function) is generally provided by a photodiode whose current is a non-linear function of the electromagnetic field.
One of the advantages of the invention is the possibility of xe2x80x9ctransportingxe2x80x9d ultrahigh frequency signals with a low attenuation per unit length by virtue of an optical xe2x80x9ccarrierxe2x80x9d. The attenuation per unit length in the fibers is in fact only of the order of 0.1 dB/km whereas it is 0.1 dB/m in an ultrahigh frequency conductor (coaxial).
According to another aspect, the laser sources can be microlasers or VCSELs (vertical cavity surface emitting lasers). These components are also compatible with a collective implementation, in the form for example of mosaics or arrays.
The device according to the invention then does not require any device for correcting the received signal.
This is because the chip laser sources (or microlasers) have a very small emission line width, of the order of a few hundred KHz, much lower than that of laser diodes (MHz) or that of VCSELs (also MHz).
Furthermore, chip lasers can be modulated (optical) emission frequency-wise with no xe2x80x9ccrossed amplitude modulationxe2x80x9d (which is not the case for laser diodes which are in general modulated frequency-wise by their supply current, and therefore also emission amplitude-wise) This frequency modification is for example obtained by electro-optical type modulation of the optical length of the microlaser cavity.
The invention also concerns an ultrahigh frequency emitting device, having:
a plurality of N laser emitter pairs, implemented in a mosaic or an array or a bar, each laser emitter pair having a first and a second laser emitter emitting at a first and a second frequency xcfx891, xcfx892, which are different,
means for slaving each laser emitter pair frequency-wise,
means for modifying the frequency of one of the laser emitters of at least one laser emitter pair with resect to the frequency of the other laser emitter of said laser emitter pair,
N means for mixing each of the beams emitted by the first emitters of the N laser emitter pairs with each of the beams emitted by the second emitters of the N laser emitter pairs and for producing a signal at the frequency xcfx891-xcfx892,
N antenna-forming means for emitting radiation at the frequency xcfx891-xcfx892.
This device also has good compactness, on account of the structure of the laser emitters in a bar or a mosaic or an array. Moreover, the use of means for modifying the frequency of one of the laser emitters with respect to the frequency of the other emitter makes it possible to obtain a slippage of the phase of one of the laser emitters with respect to the phase of the other laser emitter. It is then no longer necessary to use specific means of varying the phases of the beams emitted by the lasers, as in the previous embodiments. Finally, the invention also concerns a radar device having an ultrahigh frequency emitter as described above.
The lasers or the laser emitters can be assembled in an array, a coupling or transmission by optical fibers being implemented between the elements making it possible to impose phase delays and the means for mixing the emitted beams.
The frequency slaving means can also be assembled in an array.
Finally, the beat signal forming means can be merged with the means for mixing either the beam emitted by the first laser and each of the N delayed beams, or each of the beams emitted by the first emitters of the N laser emitter pairs with each of the beams emitted by the second emitters of the N laser emitter pairs and delayed by the elements making it possible to impose a phase delay.
According to another aspect, the lasers or the laser emitters are assembled in an array and multiplexed by a multiplexer, an optical fiber connecting the multiplexer and a demultiplexer.
For this, laser sources shifted optical frequency-wise, by the multiplexing step, can be implemented. To that end, an adjustment of the cavity length is implemented: each laser cavity has, for example, associated with it a Bragg grating type mirror implemented on a corresponding guide of the multiplexer.
The invention therefore also concerns an optical device having:
lasers or laser emitters (microlasers or laser diodes) implemented in a mosaic or an array,
a multiplexing device having integrated optical guides, each optical guide corresponding for example to a laser source or a laser emitter or a laser cavity,
a Bragg grating type mirror, implemented (for example: etched) on each guide of the multiplexing device.